1. Field of the Invention
The present invention relates to structural foams made of unsaturated polyester resins and to processes for preparing same.
2. The Prior Art
The term "unsaturated polyester resins" as used herein means those resins obtained by the polymerization of a mixture of an unsaturated polyester, formed by the polycondensation of at least one .alpha.,.beta.-ethylenically unsaturated dicarboxylic acid with a polyvalent alcohol, and an ethylenically unsaturated monomer copolymerizable therewith. Some examples of ethylenically unsaturated dicarboxylic acids are maleic, fumaric, itaconic and mesaconic acids. Along with the unsaturated dicarboxylic acids it is possible to employ mono or polyfunctional saturated aliphatic carboxylic acids, such as adipic, succinic and glutaric acid and the like; mono or polyfunctional aromatic carboxylic acids, such as phthalic, isophthalic, terephthalic, benzoic acids etc., and anhydrides such as the phthalic anhydrides and the trimellitic anhydrides.
As polyvalent alcohols it is possible to use ethylene glycol, propylene glycol, 1,2 butane diol, diethylene glycol, 1,6 hexane diol, trimethylol propane, pentaerythritol, glycerol, neopentylglycol, 1,4-cyclo-hexane-diol, etc.
Unsaturated monomers suitable for being used are those containing an .alpha.,.beta.-ethylenically unsaturated bond and which are capable of polymerizing with the unsaturated polyester. Such unsaturated monomers are for example the copolymerizable aromatic hydrocarbons, such as styrene and alkyl derivatives thereof, e.g., methyl-styrene, vinyl-toluene, divinyl-benzene, etc.; esters of acrylic or methacrylic acid with alcohols containing 1 to 8 carbon atoms, such as methyl acrylate, ethyl acrylate, butyl acrylate, and the corresponding esters of methacrylic acid; amides such as acrylamide and methacrylamide; N-methylamides such as N-methyl-acrylamide; etherified N-methylamides such as N-butoxy-methylacrylamides; nitriles of .alpha.,.beta.-unsaturated monocarboxylic acids, such as acrylonitrile and methacrylonitrile; vinyl esters of mono or polycarboxylic acids, such as vinyl acetate, vinyl propionate, divinyl succinate, etc.; N-vinyl compounds such as vinyl pyrrolidone; vinyl ethers of mono or poly-alcohols, such as isobutyl-vinyl-ether; allyl compounds such as the allyl esters of saturated or unsaturated mono or poly-carboxylic acids, such as diallyl-phthalate, diallyl-maleate, diallyl-isophthalate, etc.; allyl ethers of mono or poly-alcohols. In general the aromatic hydrocarbons are the most often used monomers. The monomeric compounds hereinabove mentioned may be used alone or mixed with one another.
Solutions of these unsaturated polyester resins in ethylenically unsaturated monomers are very difficult to foam so as to form structural foams, because they retain only with great difficulty air or inert gases entrapped in a regular and stable manner.
With a view toward overcoming this drawback it has heretofore been suggested to utilize expanding agents, either of the chemical or physical type, such as, for example, isocyanates or Freon.RTM., which are added to the unsaturated polyester resin at the moment of foam formation.
Such agents, however, have not led to satisfactory results because the copolymerization temperature of the unsaturated polyester resin solutions in ethylenically unsaturated monomers begins to rise very slowly. As a result of this, the resin undergoes gelation well before the temperature is high enough to completely activate the expanding agent.
It has also been suggested to add to the unsaturated polyester resin solution an additive capable of modifying the surface tension of the resin in contact with air, i.e., an air-resin surfactant, such as piperidine, maleic anhydride, tetrahydrophthalic anhydride, urea, melamine and resins thereof, thiocarbamates, butylcarbamate, etc., and then to entrap the air into the mass by means of a mixing device. Still, in order to succeed in obtaining acceptable density values of the foams, i.e., of the order of 0.3, corresponding to 300 kg./cm.sup.3, these surfactants need to be added in amounts generally not less than 10% by weight.
However, so high an amount of additive causes, in general, a deterioration of the mechanical characteristics of the structural foams eventually obtained. Furthermore, many of these additives are converted to the gaseous state during the cross-linking, and this causes, both in closed and open molds, a quite non-homogeneous size distribution of the resulting bubbles.
Finally, some of these additives interract with the components of the catalyst system, thus slackening the cross-linking kinetics or, as in the case of the anhydrides, the moisture forms highly corrosive acids.
It is an object of this invention to provide a resin capable of entrapping air or other inert gases, without the above-described disadvantages.